Draw a white  spiky circle in a separate layer , and blur it out. Try with few irregular ones one over another with reduced layer opacity and play with blending options  ,but  I think normal mod will work here.

If it doesnt look too good try adding a layer mask , and use a radial gradient on it , so you will get fading effect.
 

You can save it as psd so you will be able to use it again without having to draw every time .

Thanks guys I will have a try then!

This particular example looks very un-natural and I wonder why anybody would want to repeat it.

I support either of the suggestions above.

I agree it looks a bit fake... Here is one of mine which sells well with natural sun

Yes I thought about it, I have to work more on my limited Photoshop skills I guess